[Builder] unable to build builder (--enable-hello-cpp-plugin requires --enable-idemm=yes)

[Felix Schwarz <felix schwarz oss schwarz eu>]

still on my quest to build builder myself ;-)

Now with the latest git (f086cb81) I get the following error when running
  ./autogen.sh --enable-tracing --enable-debug --enable-hello-cpp-plugin

...
checking for thread-safe support in libgit2... yes
checking for SSH support in libgit2... yes
configure: error: --enable-hello-cpp-plugin requires --enable-idemm=yes

(I got the line from the ".buildconfig" file in builder's git).

This looked like a trivial error (just have to add "--enable-idemm=yes") but
unfortunately adding this parameter to the autogen.sh invokation doesn't
change anything.

This happens when building inside Flatpak (builder nightly) and when running a
"native" build from F25.

Any ideas?
Felix